Where the red channel of the texture is more ‘on’ ( ie, white ), it will give a number closer to 1, otherwise .2
So, it’s taking a value that is 0-1 and mapping it to -.2 to 1. Which makes no sense, because it’s then clamping it to 0-1 again…
It’s like a desaturate and it cuts out some levels - why does it make no sense?
Well, why bother using a 0-1 to lerp between -.2 and 1, and then clamping it back to 0-1? Also, I though anything under zero ended up as zero anyway?..
Could very well be me… 
Because.
It’s basically creating actually black areas in the image.
Think about it this way.
Take a render cloud, make the cloud slightly darker.
If you do this in photoshop the end result is already in 0 to 1 range. Not the same in engine.
So, if you were to ADD that without the clamp to something, you’ll be effectively making that something -2 in some areas - which is unpilely to be the needed result.
Particularly when you then multiply it by something else.
Whereas X by 0 is?
Ok, reducing info, basically…
Yes.
We all prefer just using the right texture 
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The “Saturate” node should be used instead of a “Clamp 0,1”.
I do this all the time.
Take a greyscale image and you can cut out levels, drop the floor, raise the cieling, etc; it’s VERY useful. If you are using a noise-based alpha, you can expand the range above 1 or below 0 and then saturate to gain/loose the impact of the lows or the highs. More extreme values + saturate are a less-expensive cheap-contrast, which essentially does the same thing anyways.
Single greyscale image can be used to fake-depth on clouds, reactive to the lighting vector and other nifty tricks:
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If I understand correctly, the compiler will do that for you under the covers.
Yeah, probably. Still, it would be more clear.